Recently, relatively smaller platforms have been developed for deep sea operations where marginal production does not merit the use of a full scale tension-leg platform (TLP). These marginal production platforms use tension-leg mooring, like conventional tension-leg platforms, but comprise smaller floatation structures. Tension-leg mooring typically comprises rigid, single-piece tendons for anchoring the structure to the sea floor, like that disclosed in Monopod TLP Improves Deepwater Economics, PETROLEUM ENGINEER INTERNATIONAL (January 1993), incorporated herein by reference. The rigid, single-piece tendons comprise a length of solid metal with buoyancy devices attached at each end. The tendons are towed to the production site and upended by flooding the lower permanent buoyancy tank. The upper permanent buoyancy tank is oversized so the tendons can be left self-standing. Also, the permanently attached buoyancy tanks make premature detachment impossible. The structure of the TLP is then ballasted by a large derrick and lowered to the previously installed tendons and then deballasted to fully tension the tendons.
Single-piece tendon systems, however, are costly to install and remove. Because single-piece tendons are inflexible, they are difficult to handle and must be buoyed and dragged from one location to another as they float on the surface of the sea. This becomes difficult in harsh weather conditions. Further, all of the tendons for a given TLP must be attached to the sea floor and the TLP must be ballasted for attachment to the tendons. TLPs are unsteady, so that it is difficult to make the connection between the free floating TLP and upended tendons. Thus, large derricks are required to stabilize the TLP for connection or disconnection to the tendons. Also, single-piece tendons only allow the TLP to be anchored at locations where the water is a specific depth because the lengths of the tendons cannot be modified.
Therefore, there is a need for tendons of variable length which may be more economically installed and removed from production sites.